The present invention relates to an improved image forming means for an image forming apparatus which forms a visual image, according to electrophotography, on an image carrying member based on image signals.
FIG. 18 illustrates one typical example of a series of multi-color image forming process for a conventional multi-color image forming apparatus. The basic process of multi-color image forming was disclosed, for example, in Japanese Patent Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 76766/1985. FIG. 18 schematically illustrates the similar process.
FIG. 18 shows changes in potential on an image carrying member, when an electrifying unit uniformly electrifies the surface of the image carrying member and then the imagewise exposing is performed with an image exposing portion. The electrifying unit and the image exposing portion constitute an electrostatic latent image forming means. PH, DA, T1 and DUP in FIG. 18 are as follows: PH represents an exposure portion on the image carrying member; DA represents a non-exposure portion on the image carrying member; T1 denotes a toner adhered to the image carrying member due to a first development; and DUP denotes change in potential caused by toner T1 adhered to the exposure portion PH due to the first development. To facilitate easy understanding, the polarity of the latent image is assumed as positive.
In Process 1, the image carrying member is uniformly electrified by the electrifying unit or the like and has positive potential E.
In Process 2, a first imagewise exposing is performed by a laser or the like, and the potential of the exposure portion decreases according to the light amount.
In Process 3, the electrostatic latent image formed as described above is developed by a developing means which is provided with positive bias virtually equal to the surface potential E of the non-exposure portion.
As a result, the positively-charged toner T1 adheres to the exposure portion PH having comparatively low potential, and forms a first toner image. The potential of the area in which the toner image is formed increases as much as DUP. However, potential of the area with the toner image normally does not reach to that of the non-exposure portion.
In Process 4, the surface of the image carrying member bearing the first toner image is electrified again by the electrifying unit. Consequently, the surface of the image carrying member is given uniform surface potential E, regardless of the presence or absence of the toner T1.
In Process 5, the second imagewise exposing is performed on the surface of the image carrying member, forming an electrostatic latent image.
In Process 6, similar to the Process 3 above, toner T2 (independent of the toner T1) is developed so as to from a second toner image. The toner T2 is also positively charged.
To form a multi-color toner image on the image carrying member, the processes mentioned above are repeated for required times. Then, the resultant multi-color toner image is transferred onto a transfer material, thereby heat or pressure is exerted, for fixing, on the transferred image. Thus, a multi-color image is formed.
Referring now to the block diagram in FIG. 19, a multi-color image forming apparatus, which has been disclosed by the present inventors, having the functions mentioned above is described in detail. The light information of a document 5 is fed into three photosensors 523, 524 and 525 via filters B, G and R from a image information input portion 502. As a result, the light information is separated into three independent color signals I.sub.B, I.sub.G and I.sub.R and entered into a recording-data generating portion 503. In the recording-data generating portion 503, recording data D.sub.Y, D.sub.M, D.sub.C and D.sub.K are formed. The recording data D.sub.Y, D.sub.M, D.sub.C and D.sub.K are generated and outputted in parallel. More specifically, yellow, magenta and cyan components of a specific pixel on a screen are simultaneously generated. Switches 5-S.sub.1 and 5-S.sub.2 are selectors to choose which of recording data D.sub.Y, D.sub.M, D.sub.C, and D.sub.K is transmitted to latent image forming means or light exposure means 521 and 522. These switches S.sub.1 and S.sub.2 are selectively driven by switching signals from the CPU 509.
Recording data (D.sub.M or D.sub.C) selected by the switch 5-S.sub.1 is transferred to the latent image forming means 521 (e.g. laser beam scanner) which promptly performs imagewise exposure to form latent image corresponding to the recording data (D.sub.M or D.sub.C). In contrast, recording data (D.sub.Y or D.sub.K) selected by the switch S.sub.2 is temporatily stored in a shift register 506. The shift register 506 transmits the stored data to a latent image forming portion 522 after a specific duration equal to a required time for the image carrying member or photoreceptor of a positive image forming means 528 to travel rotatively the distance from the latent image forming means 521 to the similar means 522. As a result, the stored recording data is transmitted to the latent image forming means 522, after a delay which corresponds with the distance between the two latent image means. According to the above arrangement, the latent images formed by the latent image forming means 521 and 522 are exactly aligned with each other so that independent color images are formed in the same position on the photoreceptor of the positive image forming portion 528. The positive image forming portion 528 develops the latent images thereon. A duplicate is completed by fixing the developed image after transferring them to a transfer sheet P.
Using this methods, an image is formed on a image carrying member based on electric signals. In this case, a laser, LED, liquid crystal shutter or similar means controlled by electric signals is used as image exposing means. This type of apparatus (digital image forming apparatus) not only has copying function, but also has advanced functions such as image storage, editing and communication. The conventional copying apparatus, which directly forms a document reflection image on an image carrying member, does not have these advanced functions.
However, the aforementioned process incurs problems. The first problem is as follows: when continuously forming many duplicates of an image from an same document by using the image forming apparatus above, the image information input portion 502, the recording-data generating portion 503 and other portions have to be repeatedly energized per duplicate, resulting in shorter service lives of elements and components constituting these portions, as well as larger power consumption for driving each portion. Furthermore, since copying time increases in proportion to the number of copies being prepared, large volume copying requires extended duration of time for completing copying. In regard to the service lives of the elements and components, power consumption and time required for copying, the apparatus above especially has a problem that all the portions (including the image information input portion 502 and record ing-data generating portion 503) have to be driven per duplicate even for a single color duplicate using one color toner.
When used as a copying apparatus, the multi-color image forming apparatus may be capable of performing high-speed image forming by parallelly accepting image data.
When entering image data from an external unit for storing, editing and communicating image data, the image data may not be parallel, unlike that transferred from an image reader, in some cases, and signals of each color component may be transmitted in the following format. FIG. 20 shows examples of the format of image data. In FIG. 20-a, Y (yellow), M (magenta), C (cyan) and K (black) are alternatively transferred per one line of an image. To identify the end of a one line data, several-bit-configured data may be incorporated as a control signal following the one line data. On the actual screen, data are arranged as described in FIG. 20-b. The data format as shown in FIG. 20-b is hereinafter referred to as the line sequential. Likewise, the color components of image data may be transferred per pixel as shown in FIG. 20-c. On the actual screen, the data are arranged as described in FIG. 20-d. (N corresponds with the number of pixels on one line in the main scanning direction.) The data format as shown in FIG. 20-d is hereinafter referred to as the dot sequential. In some cases, independent color components on one screen may be alternatively transferred per screen. This type is hereinafter referred to as the screen sequential. These types of image data are completely different from that entered by the image reader. Therefore, data processing function shown in FIG. 19 cannot cope with the above-mentioned format of image data.